Bottom Line:
The macrolides were found to be bound close to the surface of the micelle.The binding to bile aggregates does not impede macrolide antibiotics from targeting bacteria.In fact, the toxicity of azithromycin towards enterotoxic E. coli (ETEC) is even slightly increased in the presence of bile, as was shown by effective concentration (EC50 ) values.

fig06: Two investigated bile mimetics: FeSSIF contains NaTC above the cmc of mixed micelles and FaSSIF below. Hence, NaTC and lecithin show more similar diffusion. The mixed micelles bind the macrolides and lower their diffusion coefficient.

Mentions:
To investigate the binding of macrolides to bile-acid aggregates under more physiological conditions, fasted- and fed-state simulated intestinal fluids (FaSSIF and FeSSIF, respectively) were used. The difference between these two solutions is based on a 1+4 dilution of FeSSIF with HBSS to obtain FaSSIF. It is suggested in the literature that lecithin and NaTC form mixed micelles with a cmc between 5–12 mm.[15] Therefore, in FaSSIF, no mixed micelles are formed, resulting in a high diffusion coefficients of monomeric bile acid (Figure 6). In contrast, the diffusion of lecithin is much slower, indicating the formation of large aggregates of this lipid. The fast diffusion of the macrolides in this solution is close to that of the same compounds in an aqueous environment, indicative that they do not bind to lecithin micelles. On the other hand, in FeSSIF, mixed micelles are formed, and can uptake the macrolides and transport them through the body. The only slight reduction of the macrolide diffusion coefficient is probably a combination of the presence of highly hydrophobic lecithin in these mixed micelles and the rather low molar ratio of the mixed micelles to macrolide. As was mentioned above, a cmc around 5–12 mm has been reported for FeSSIF. The concentration of NaTC is 15 mm, so 3–10 mm are found in micelles. The aggregation number of pure NaTC micelles is approximately eleven. If it is similar in mixed micelles, this would result in a mixed micelle concentration between 0.3 and 1 mm. It should also be mentioned that in vivo, a concentration of 1 mm macrolide would be extremely high. At lower concentrations, the ratio of macrolide bound is probably much higher. It is surprising that the macrolides do not interact with the lecithin aggregates, which would lead to lower D values of them in FaSSIF medium. Binding to bile acid and mixed micelles clearly does not only occur through hydrophobic interactions in the micelle, but has to involve some other interactions as well. Macrolides contain a number of polar groups, which clearly prevent them from binding to highly hydrophobic aggregates. To get a better idea about the exact mode of binding, it is important to know the topology of the macrolides in the micelles.

fig06: Two investigated bile mimetics: FeSSIF contains NaTC above the cmc of mixed micelles and FaSSIF below. Hence, NaTC and lecithin show more similar diffusion. The mixed micelles bind the macrolides and lower their diffusion coefficient.

Mentions:
To investigate the binding of macrolides to bile-acid aggregates under more physiological conditions, fasted- and fed-state simulated intestinal fluids (FaSSIF and FeSSIF, respectively) were used. The difference between these two solutions is based on a 1+4 dilution of FeSSIF with HBSS to obtain FaSSIF. It is suggested in the literature that lecithin and NaTC form mixed micelles with a cmc between 5–12 mm.[15] Therefore, in FaSSIF, no mixed micelles are formed, resulting in a high diffusion coefficients of monomeric bile acid (Figure 6). In contrast, the diffusion of lecithin is much slower, indicating the formation of large aggregates of this lipid. The fast diffusion of the macrolides in this solution is close to that of the same compounds in an aqueous environment, indicative that they do not bind to lecithin micelles. On the other hand, in FeSSIF, mixed micelles are formed, and can uptake the macrolides and transport them through the body. The only slight reduction of the macrolide diffusion coefficient is probably a combination of the presence of highly hydrophobic lecithin in these mixed micelles and the rather low molar ratio of the mixed micelles to macrolide. As was mentioned above, a cmc around 5–12 mm has been reported for FeSSIF. The concentration of NaTC is 15 mm, so 3–10 mm are found in micelles. The aggregation number of pure NaTC micelles is approximately eleven. If it is similar in mixed micelles, this would result in a mixed micelle concentration between 0.3 and 1 mm. It should also be mentioned that in vivo, a concentration of 1 mm macrolide would be extremely high. At lower concentrations, the ratio of macrolide bound is probably much higher. It is surprising that the macrolides do not interact with the lecithin aggregates, which would lead to lower D values of them in FaSSIF medium. Binding to bile acid and mixed micelles clearly does not only occur through hydrophobic interactions in the micelle, but has to involve some other interactions as well. Macrolides contain a number of polar groups, which clearly prevent them from binding to highly hydrophobic aggregates. To get a better idea about the exact mode of binding, it is important to know the topology of the macrolides in the micelles.

Bottom Line:
The macrolides were found to be bound close to the surface of the micelle.The binding to bile aggregates does not impede macrolide antibiotics from targeting bacteria.In fact, the toxicity of azithromycin towards enterotoxic E. coli (ETEC) is even slightly increased in the presence of bile, as was shown by effective concentration (EC50 ) values.